Nondestructive inspection (NDI) of structures involves thoroughly examining a structure without harming the structure or requiring significant disassembly of the structure. Nondestructive inspection is typically preferred to avoid the potential for damaging the structure. Nondestructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. For example, non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to, or flaws in, the structure. Inspection may be performed during manufacturing of a structure and/or once a structure is in service. For example, inspection may be required to validate the integrity and fitness of a structure for continued fitness for use in subsequent manufacturing and\or assembly processes, as well as for future ongoing use in service, either periodically or subsequent to an event potentially causing damage to the structure.
Among the structures that are routinely nondestructively tested are composite structures, such as composite sandwich structures and other adhesive bonded panels and assemblies. In particular, composite structures are commonly used throughout the aircraft industry because of the engineering qualities, design flexibility and low weight of composite structures, such as the stiffness-to-weight ratio of a composite sandwich structure. It is frequently desirable to inspect composite structures to identify any flaws, such as cracks, voids or porosity, which could adversely affect the performance of the composite structure. For example, typical flaws in composite materials or composite sandwich structures, which are generally made of one or more layers of lightweight honeycomb or foam core material with composite or metal skins bonded to each side of the core, may include defects in bonds that occur at the interfaces between layers of the composite materials or between the core and the composite skins.
Various types of sensors may be used to perform nondestructive inspection. One or more sensors may be moved over or along the portion of the structure to be examined, and receive data regarding the structure. For example, a pulse-echo, through-transmission, or shear wave sensor may be used to obtain ultrasonic data regarding the structure, such as for thickness gauging, detection of laminar defects and porosity, and/or crack detection in the structure. Resonance, pulse echo, or mechanical impedance sensors may be used to provide indications of voids or porosity, such as in adhesive bond lines of the structure. High resolution inspection of aircraft structure are commonly performed using semi-automated ultrasonic testing (UT) to provide a plan view image of the part or structure under inspection. While solid laminates may be inspected using one-sided pulse echo ultrasonic testing, composite sandwich structures typically require through-transmission ultrasonic testing to achieve high resolution inspection. In through-transmission ultrasonic inspection, ultrasonic sensors such as transducers, or a transducer and an associated receiver sensor, are positioned in mutually facing relationship, contacting opposite sides of the structure to be inspected such as opposite surfaces of a composite material. An ultrasonic signal is transmitted by at least one of the transducers, propagated through the structure, and received by the other transducer or receiver sensor.
Ultrasonic testing generally requires a coupling material (e.g., a fluid, such as water) to aid transmission of the ultrasonic energy to the test specimen because the acoustic impedance mismatch between air and solids (i.e., the test specimen) is unacceptably large. This mismatch causes reflection of the sound waves and a loss in scan quality if a coupling material is not used. In order effectively pass the ultrasonic signal to the structure to be inspected, the coupling material is generally required to be uniform in order to successfully pass the signal through the coupling material without altering the signal. For example, when a flowing fluid, such as water, is utilized as a coupling material, flow of the fluid should be in a substantially laminar regime in order to effective pass the ultrasonic signal to the structure to be inspected without substantially altering the ultrasonic signal.
Furthermore, ultrasonic testing may be limited in terms of inspection speed as many sensors scan only one point or one small area at time. In an ultrasonic inspection involving scanning only one point or one small area at time, the amount of time and resources spent to inspect relatively larger structures, such as aircraft structures, may be undesirable or impractical.